In recent years, for the purpose of wireless communication such as a keyless operation system for a vehicle, an antenna device having a linear element has been utilized. As an antenna device utilizing a linear element, a monopole antenna on which a wire element with a ¼ length of an antenna operation wavelength disposed to a ground plate has been conventionally and generally used. However, since the overall size and height of the monopole antenna are large and high, an inverse L-type antenna in which the monopole antenna is bent in the middle so as to reduce the size and height thereof is being developed.
Furthermore, in the inverse L-type antenna, a reactance section, which is determined by the length of the horizontal portion of an antenna element parallel to the ground plate, is capacitive and becomes a large value, thereby making it difficult to match with respect to a 50Ω power feed line. Hence, in order to facilitate the matching between the antenna element and the 50Ω power feed line, a so-called inverse F-type antenna has conventionally been devised. The inverse F-type antenna is configured such that a stub which connects the ground plate to a radiation element is provided near a power feed point provided in the midway of the antenna element.
With this arrangement, it is easy to cancel out the capacitive reactance created by the reactance section so as to achieve matching with the 50Ω power feed line. For example, Japanese Unexamined Patent Application Publication No. 2006-197528,the entire contents of which is hereby incorporated by reference, discloses an inverse F-type antenna which is applicable to a folded-down portable wireless apparatus and which has an antenna element that is bent so as to be perpendicular to a flexible flat cable that is disposed on a printing wiring substrate and connected to a printing wiring substrate. In the inverse F-type antenna, the antenna element is also folded back in the vertical direction with respect to the printing wiring substrate.
However, even in the conventional technique described above, problems still remain. Specifically, in the conventional inverse F-type antenna, although a stub is provided near the power feed point in the midway of the antenna element, the occupation area of the matching section is enlarged, resulting in difficulty in attaining the reduction in size of the antenna. In addition, since the characteristics of the matching section with respect to the antenna element may be deteriorated, directivity is thereby limited, resulting in a disadvantage in which only one of polarization in horizontal plane and polarization in vertical plane is improved. Furthermore, although the inverse F-type antenna disclosed in Japanese Unexamined Patent Application Publication No. 2006-197528 described above improves the antenna characteristics by bending the length and width of the antenna element with respect to the flexible cable, such improvement depends on a surrounding environment. Thus, not only polarization and directivity cannot be improved, but also miniaturization and thinning are difficult due to its structure.
Accordingly, it would be advantageous to provide an antenna device that improves polarization and directivity and enhances further miniaturization and thinning.